1. Technical Field
The present invention relates to a shift control device for a continuously variable transmission, and more particularly relates to a shift control device for a continuously variable transmission which is installed in a vehicle equipped with a drive assist device, and which is capable of switching over control modes in accordance with driver operations.
2. Related Art
These days, vehicles are in practical use which enable selection (switching) among multiple operating modes, for example engine output characteristic (output mode), continuously variable transmission gearshift characteristic and/or gearshift modes, in accordance with a user preference (see Japanese Unexamined Patent Application Publication (JP-A) No. 2012-192843, for example).
An example of such vehicles is a vehicle which allows the driver to select an engine output characteristic among three output modes, according to preference. The three engine output modes are a normal mode suitable for normal driving, a save mode (or economy mode) where output torque is suppressed to conserve gas mileage, and a power mode which gives priority to power, having an output characteristic with good responsivity from low speed to high speed ranges. The vehicle also enables the shift control mode of the continuously variable transmission to be selectively switched between a normal stepless shift control mode where gear ratio is steplessly changed according to the running state of the vehicle, such as accelerator position and vehicle speed, and a stepped shift control mode where the gear ratio is changed like with a stepped automatic transmission (AT), in accordance with the selected output mode. For example, normal stepless shift control is executed when the save mode or normal mode is selected, and stepped shift control which performs stepped acceleration is executed when the power mode is selected.
Further, drive assist devices are in practical use which assist driving operations of the driver, by recognizing the driving environment in front of the vehicle and emitting a warning regarding an obstacle ahead, detecting a preceding vehicle from the recognized driving environment ahead and performing preceding vehicle following control or warning control (e.g., see JP-A Nos. 2012-192843 and 2012-206699. In particular, JP-A No. 2012-206699 discloses a technique for adaptive cruise control (ACC) having a preceding vehicle following function. Specifically, the vehicle is driven at a constant speed if no vehicle is detected ahead, and if a preceding vehicle is detected, the vehicle is controlled to follow the preceding vehicle.
When the vehicle is controlled to follow the preceding vehicle such that a distance to the preceding vehicle is kept constant, the drive assist device such as described above may be configured to calculate required driving force which is necessary for the vehicle to follow the preceding vehicle, output required engine torque to the engine based on the computation result, and output to the continuously variable transmission a required gear ratio or a required speed (e.g., required turbine speed) which is defined with the required gear ratio.
When the above-described power mode, i.e., stepped shift control mode, is selected in a vehicle equipped with a continuously variable transmission capable of switching between driving modes (shift control modes) such as described above, the gear ratio which the stepped shift control can assume is restricted. Thus, the target gear ratio calculated by continuously variable transmission (or the target speed obtained from the target gear ratio) may not be the same as the required gear ratio (or the required speed obtained from the required gear ratio) calculated by the drive assist device. Accordingly, when the vehicle is following the preceding vehicle according to instructions of the drive assist device (required engine torque and required gear ratio), and then the preceding vehicle leaves the road so the driver of the vehicle equipped the drive assist device steps on the accelerator pedal, for example, stepped acceleration may not be performed against the driver's intention of stepped acceleration, thereby giving the driver a feeling of wrongness.